Various communications protocols are used for cellular and other wireless communications. One such protocol is Code Division Multiple Access (CDMA), which employs spread-spectrum technology and uses unique codes assigned to different signal streams to allow the signals to share a common frequency band. A third generation (3G) variant of CDMA is Wideband Code Division Multiple Access (W-CDMA), which is a wideband spread-spectrum mobile air interface that utilizes the direct sequence CDMA signaling method and achieves higher speeds and support more users, as compared to typical Time Division Multiplexing (TDMA) used by second generation (2G) Global System for Mobile Communications (GSM) networks, for example.
Some wireless environments may also be subject to high Doppler frequency offset or spreading, as well as multi-path fading. Thus, to achieve desired signal acquisition may require relatively sophisticated frequency tracking circuitry. One particular receiver that is commonly used to mitigate the effects of multi-path fading is the so-called RAKE receiver. A RAKE receiver uses several sub-receivers each delayed slightly to tune in to the individual multi-path components. Each component is descrambled and de-spread independently, but combined at a later stage.
One exemplary approach to acquire and track pilots in a CDMA system which utilizes a RAKE receiver arrangement is set forth in U.S. Pat. No. 7,088,955 to Challa et al. Frequency acquisition of a number of signal instances (i.e., multi-paths) in a received signal is achieved concurrently based on a frequency control loop maintained for each finger processor of a RAKE receiver. Upon successful acquisition, frequency tracking of acquired multi-paths is achieved based on a combination of a frequency control loop maintained for an oscillator used for downconverting the received signal and the RAFCs for the finger processors. In a tracking mode, the VAFC tracks the average frequency of the acquired multi-paths by adjusting the frequency of the oscillator. The RAFC of each finger processor tracks the residual frequency error (e.g., due to Doppler frequency shift) of the individual acquired multi-path by adjusting the frequency of a complex sinusoidal signal used in a rotator within the finger processor.
Despite the potential advantages of such systems, further advancements may be useful to help reduce the complexity of receiver architectures, yet while still providing desired pilot signal acquisition, in relatively harsh multi-path environments, even when characterized by a high Doppler frequency or frequency offset.